On July 23, 2018, a notable disaster occurred In SE Asia, leading to the confirmed death of at least 40 persons, with many more missing and 6,600 homeless. This tragedy was not a natural event but rather a failure of a just-filled hydroelectric dam on a mist-shrouded plateau of Southern Laos. The failure sent a half-billion cubic meter cascade of muddy water over a 200 meter formerly scenic waterfall, obliterating six villages below the dam. The disaster occurred in the midst of what some have called a hydropower rush in that small mountainous country, raising questions not only about the care with which these short-lived projects, developed by aggressive teams of international investors and contractors, are being built, but also about the suitability of Laos' tropical geography for engineering concepts largely imported from western countries.

Richard Meehan, who began his engineering career designing and building dams nearby in Thailand and who has taught courses for many years at Stanford's School of Earth Sciences and also at Blume Earthquake Engineering Center, has been investigating the dam's failure from Bangkok using new space technologies. His report on the progress of his work follows.

The July 2018 Laos Dam Failure: a preliminary geotechnical diagnosis

Richard Meehan

The world-wide ground elevation dataset SRTM 1-arc second (ground elevations on a 30 meter grid for most of the earth), made available in 2015 by NASA, finds applications in understanding of global hazardous conditions and disasters, new and old. The recent (July 23, 2018) hydroelectric dam failure at the Xe-Pian Xe-Namnoy Project on the Bolaven Plateau in Laos serves as an example of a need for timely failure diagnosis. Within hours after the failure of the billion-dollar project conflicting explanations were announced by the multinational partners including Thai investors and Korean contractors who built and began to operate the project on a remote plateau in Southern Laos. Half of those involved blamed the failure on excessive rain, the others on substandard saddle dam construction. Though the Lao Prime Minister, representing the Lao PDR’s membership in the partnership, called for a collaborative effort by world experts to find out the truth about the accident, requests for any information on the design, construction, and startup operations of the dam have not been granted by anyone involved. As of the end of September 2018, two

months after the failure, official investigations had reportedly not yet been started in part because of difficulty in accessing the high plateau but also because of the unexpected appointment of independent investigators by various parties and their lawyers suggesting the likelihood of a prolonged adversarial investigations. Meanwhile, the future of this and other problem-plagued hydro projects in Laos hangs in the balance, with hasty attribution of the failure to extraordinary weather or local specific defects in dam construction providing little guidance for urgent policy decisions which the Lao Prime Minister had recognized in his call for the truth. But satellite ground elevation data and imagery, when combined with readily available archival information from mineral explorations, along with experience on dam projects in similar terrain in nearby Northeast Thailand and in other comparable areas of tropical soils, show that this extended western arm of the reservoir was built on a sinkhole -- a dry basin (Fig. 1) that probably never, even over millennia, had retained any of the heavy (1-4 meters) summer rainfall that drenched the plateau, famous for coffee growing and waterfalls.

Thus the new and immediately available global satellite data allow for a synthesis of failure explanations into a hypothesis that is both specific to the project but also carries broader implications for this type of development. The failure hypothesis goes as follows: on the first filling of the reservoir, a wave of groundwater from the main reservoir pushed southwest eventually filling the dry basin behind the saddle dam with 20 m of water. The progress of the underground wave can be tracked by rising springs visible on cloud-piercing satellite images from the Sentinel 1 satellite. Rising water pressures, still within the normal operating specifications for the project but unprecedented in geological time, enlarged underground free-draining conduits within the naturally fragile basalt ridge line that supported the new saddle dam. With a rising water level and loss of support, the brittle earth fill dam began to sink into the void and crack extensively. The rising reservoir then cascaded over or through the fragmented crest, washing away the remnants of the dam and fifteen meters of erodible foundation and spilling, late at night, half a billion cubic meters of water and debris. This muddy flood plunged over the precipice of a downstream touristic waterfall onto small villages hundreds of meters below with tragic consequences. Six villages were completely destroyed with thousands of people left homeless. Two months later 40 people were confirmed dead, but 65 were still missing. The process of the failure proposed here is illustrated with photos and graphics following (Figs 2-4).

Aside from the telltale dry basin, other known risk factors were present: geological formations with known high subsurface water mobility and storage (Viossanges) including presence of volcanic lavas (Japan International Cooperation Agency) with high leakage potential at both local (soil-filled joints) and larger scale (collapse features such as lava tubes, caves and possibly exploded features from flash water vaporization); an interface between lava and very old sedimentary rocks with weathered cracks and voids along with stress conditions favoring expanded flows under increased pressure (Longdren, Hamilton); and tropical soil weathering profiles (Douangsavanh, Sanematsu, SARCO) long known by dam builders for their instability (ICOLD, Blight). The dam embankment consisted almost entirely of fill composed of local residual soils susceptible to rapid internal erosion and disintegration when cracked and subjected to high water pressures. These high risk factors raise the question of whether this type of tropical geography is suitable for the aggressive and loosely managed "hydropower rush" that is now underway in remote regions of Laos. Whether the predictable foundation and water retention problems advanced were ever investigated or considered during construction of the